Working Paper	ARAC/87/WP/9
	July, 1987

Investigations on acid sulfate soils of the brackishwater experimental fish farm, Buguma, Rivers State

C. O. Dublin-Green

AFRICAN REGIONAL AQUACULTURE CENTRE, PORT HARCOURT, NIGERIA
CENTRE REGIONAL AFRICAIN D'AQUACULTURE, PORT HARCOURT, NIGERIA

UNITED NATIONS DEVELOPMENT PROGRAMME
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH
PROJECT RAF/82/009

July, 1987

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INVESTIGATIONS ON ACID SULFATE SOILS OF THE BRACKISHWATER EXPERIMENTAL FISH FARM, BUGUMA, RIVERS STATE

C. O. DUBLIN-GREEN

ABSTRACT

A study conducted on the soils of the brackishwater experimental fish farm Buguma, showed that the area contains potential acid-sulfate materials within 12cm of pond bottom in the soil profile. The sulfate sulfur (SO₄-S) content of the sulfidic horizon is 2,700ppm. High values of exchangeable acidity and Al were also recorded for this horizon. Values were 16.96 and 7.89 meq/100g of soil respectively.

The soils are saline, characterised by high soluble salt content and high values of K, Ca and Mg ions. pH values in the wet state ranged between 7.7 and 6.9, but on air drying, values went down to as low as 3.1. Total nitrogen and available phosphorus were relatively low, values ranged from 0.07 – 0.35% and 1.40 – 7.08 ppm respectively. Lime requirement was calculated for the soils; results indicate the need for approx. 4.1 tonnes of agricultural lime per hectare for neutralization.

RECHERCHES SUR LES SOLS SULFATES ACIDES DE LA
FERME PISCICOLE DE BUGUMA, RIVERS STATE

C.O. DUBLIN-GREEN

RESUME

Une étude des sols de la ferme piscicole en eau saumatre de Buguma a montré qu'ils étaient potentiellement des sols sulfates acides sur une profondeur de 12 cm du profil du fond des étangs. La teneur en sulfates (SO₄-S) dans l'horizon de surface est de 2700 ppm. Des valeurs élevées d'acidité échangeable et d'alumunium ont été rélevés dans cet horizon. Les valeurs étaient de 16.96 et 7.89 meq/100g de sol, respectivement.

Les sols sont salins et caracterisés par des teneurs élevées en sels solubles et en ions K, Ca et Mg. Les valeurs de pH du sol humide se situent entre 7.7 et 6.9, mais au séchage à l'air, ces valeurs descendent jusqu'à 3.1. Les valeurs obtenues pour l'azote totale et le phosphore disponible étaient bas, les valeurs se situent entre 0.07 à 0.35% et 1.40 à 7.08 ppm respectivement. La quantité de chaux nécessaire pour neutraliser les sols a été calculée: approximativement 4.1 tonnes de chaux agricole par hectare sont nécessaires.

INTRODUCTION

Over one million hectares of unclassified wetlands in the mangrove swamp of the Niger Delta is reported to be affected by acid sulfate conditions (Brady, 1981; Nsirimah et al, 1985). These soils are derived from marine and alluvial sediments and they contained large amount of undecomposed fibrous roots of the red mangrove (Rhizophora spp). Acid sulfate soils developed as a result of the drainage of parent sediments that are rich in sulfidic materials.

The formation of acid sulfate soils in mangrove swamps is favoured by the abundant supply of sulfates (from sea water) and organic matter (Singh, 1980). Decomposition of organic matter depletes the oxygen, giving rise to anaerobic conditions and thus activating sulfur reducing bacteria. These bacteria decompose the organic materials and at the same time utilize the sulfates brought in by tidal influx for their respiratory processes and thus producing sulfides (Pons, 1973). The resulting sulfides get oxidised under aerobic conditions producing free sulfuric acid, iron and aluminium ions.

Difficulties encountered in the utilization of these soils for agricultural purposes are mainly due to the acidic conditions. Utilization of acid sulfate soils in brackishwater aquaculture has been extensively studied in South and South-East Asia, Malaysia and Philippines (Brinkman and Pons, 1972; Singh, 1980; Simpson et al, 1983; Neue and Singh, 1983; Simpson and Pedini, 1985). Here in Nigeria these soils and their utilization have been subjected to very little study. The Buguma brackishwater experimental fish farm was established by the Nigerian Government for carrying out investigations, development and management of brackishwater fish culture. Research activities on the farm have been focused on the culture and production of Tilapia guineensis, S. melanotheron, and grey mullets (Mugil spp). The objective of this study therefore, is to provide relevant soil data necessary for a better understanding and management of these soils.

MATERIALS AND METHODS

Description of Study Area

The brackishwater fish farm is located near Buguma in a fairly extensive swamp of the Buguma creek. The tidal range in the creek is about 1.5m, this high tidal range makes it possible to fill the ponds at high tide and drain at low tide (Pillay, 1965). The mean annual rainfall in Buguma is about 2022mm (Isirimah et al, 1985). The vegetation of the area is dominated by the red mangrove (Rhizophora spp). Salinities of creek water ranged between 9.2‰ in the rainy season and 19‰ in the dry season.

Fig. 1: The brackishwater experimental fish farm, Buguma, Rivers State

The fish farm has 15 ponds of various sizes (0.2 – 1 ha). In addition to these, there are 7 nursery ponds of various sizes (Fig. 1). Soil tests conducted during the construction of the ponds in 1965 revealed that the soils were acidic. A pH of 5 was recorded for wet soils from the pilot pond (Pillay, 1965). The magnitude of this problem was however not realised until fish mortalities were recorded in the pilot pond soon after a heavy rainfall. Nair (1969) also reported mass mortalities of fish from pilot pond soon after a heavy rainfall. pH values found to have lowered appreciably after heavy rains. In both cases, the acidic conditions was arrested by extensive liming, but the problem however persisted. A more recent case of mass mortality of stocked Tilapia guineensis was reported in pond K by Campbell (Personal communication, 1986). The pH of the pond water remained at 3.5 despite extensive application of agricultural lime.

Sampling of Soils

The study was conducted between January and March, 1986. Most of the ponds except P and Q were drained at the time of this study. The bottom of the ponds were quite dry with several cracks after six weeks of exposure.

A soil profile pit was dug in pond K, while soil samples were obtained from various depths in pond I₁ using a soil - auger. Samples were also collected from ponds P, Q and their dykes. The soils were described using the FAO (1977) guidelines for soil description.

Analyses of Soil Samples

The soil samples were kept moist by packing inside black polythene bags. In the laboratory, soil pastes (1: 1 soil to water) were prepared for immediate measurement of soil pH. pH determination was carried out on duplicate samples using a glass-calomel combination electrode pH-meter, standardized with buffer solutions of know pH. A sub-sample was air dried and pH monitored at 5 days interval for 40 days.

Soluble salt content was determined by measuring the electrical conductivity (EC) of saturated extracts prepared from soil pastes. The particle size distribution was analysed by the hydrometer method following the removal of organic matter (McKeague, 1978). Organic matter and carbon were determined by the Walkey and Black method. Total nitrogen was analysed using the semi-micro-Kjeldal method while available phosphorus was determined by the Bray and Kurtz No. 1 method (McKeague, 1978).

Cation exchange capacity (CEC) was determined by sodium acetate method (NaOAC). This method has been recommended for the determination of CEC of saline soils because these soils have the tendency to fix NH⁴ when NH₄OAC method is used (McKeague, 1978). The concentration of Ca and Mg ions in the NaOAC extract collected from the CEC experiment was determined using EDTA titration. K ions from the extract was determined by flamephotometry (McKeague, 1978). Exchangeable acidity and A1 were determined by replacement with 1 normal KCl, the extract was then titrated with NaOH. Sulfate-sulfur (So₄-S) was extracted by KH₂ PO₄ method. The concentration of SO₄-S) in the extract was determined by turbidimetric method. The lime requirement of the soil was calculated using the buffer technique. The buffer solution was prepared from P - nitrophenol, calcium-acetate and mgO. The mixture was adjusted to pH 7 (Swingle, 1964).

RESULTS

Physical Properties

The soils of the fish farm are classified as (Fibric) sulfishemist because they contain fibric materials with over 80% mangrove fibers which reduce to over 60% after rubbing (Soil Taxonomy, 1975). The soils have low bulk density due to the high amount of fibric materials and other partly undecomposed organic matter.

The horizon arrangement in the soil profile was A, Oi, C1 and C2. The A-horizon contained a prominent olive (5Y 5/3) mottled layer due to the presence of jarosite. This layer is referred to as the sulfidic horizon. The soil profile has a loamy sand texture in all the horizons except the A-horizon (0 – 12cm) which has a sandy loam texture (Table I).

The dyke soils are very dark grey in colour with some fibric materials. A striking characteristic observed on the coloured mottles (2.5Y 6/6) of jarosite. This is a potentially dangerous mineral. Hydrolysis of jarosite is responsible for the release of acidic waters into the fish ponds during heavy rainfall.

TABLE I

Some physical properties of soils from the brackishwater experimental fish farm, Buguma

Sample Location	Depth Cm	Colour (Munsell)	T E X T U R E
Sample Location	Depth Cm	Colour (Munsell)	Sand %	Silt %	Clay %	Class
Pond K	0 – 12	5Y 4/1 mottled; (5Y 5/3)	68.0	30.9	0.7	Sandy loam
Pond K	12 – 28	7.5YR 3/2	84.0	4.6	11.4	Loamy sand
Pond K	28 – 57	10YR 6/2	84.7	4.3	10.9	Loamy sand
Pond K	57 – 113	10YR 6/2	84.7	2.4	12.9	Loamy sand
Pond K	113 – 154	10YR 5/1	84.7	4.6	10.7	Loamy sand
Pond K	154 – 169	10YR 5/1	85.7	3.3	10.9	Loamy sand
Pond I₁	0 – 30	5Y 4/1	41.5	52.6	5.9	Silt loam
Pond P	0 – 20	2.5Y 4/2	63.5	22.2	14.3	Sandy loam
Pond Q	0 – 20	2.5Y 4/2	65.3	28.2	6.5	Sandy loam
Dyke Soil Pond P	Suface	mottled; 2.5Y 6/6	-	-	-	-

Chemical Properties

pH determination for wet and dry soil states showed the tendency of all samples to develop extreme acidity on air drying. The pH in the wet state ranged between 6.95 – 7.70 but on air-drying for 40 days, the pH gradually went down to less than 3.8 in all samples (Table II). The soils of production ponds P and Q were found to be more acidic than others. The pH of the dry soil was as low as 3.10. pH values recorded for dyke soils were very low, values ranging between 2.05 - 1.90.

The soils are saline due to daily influx of saline tidal water from the creek. The saline nature is shown by the high EC values of saturated extracts of soil samples. The highest value of 125m mhos/cm was recorded in the A-horizon of the soil profile. High values of exchangeable acidity/aluminium, and sulfate - S were also recorded for this horizon.

TABLE II

Effect of air-drying on pH of soil from the Buguma fish farm (see text for details)

Sample Location	Depth cm	DAYS OF AIR-DRYING
Sample Location	Depth cm	0	4	8	12	15	19	25	29	35	40
Pond K	0 – 12	7.20	6.50	4.95	4.50	4.30	4.10	4.10	4.10	3.90	3.70
Pond K	12 – 28	7.50	6.35	4.50	4.10	4.00	3.70	3.40	3.40	3.10	2.90
Pond K	28 – 57	7.60	6.95	5.50	5.00	4.70	4.30	3.90	3.80	3.40	3.40
Pond K	57 – 113	7.70	7.15	5.90	5.55	5.20	4.90	4.40	4.30	3.80	3.60
Pond K	113 – 154	7.60	6.65	5.60	5.20	5.00	4.70	4.40	4.40	3.90	3.20
Pond K	154 – 169	7.45	7.10	6.10	5.70	5.60	5.30	5.00	4.90	4.40	3.70
Pond I₁	0 – 30	7.10	5.85	5.15	4.80	4.70	4.50	4.30	4.00	3.80	3.60
Pond P	0 – 20	6.95	6.35	4.90	4.50	4.40	4.10	3.70	3.70	3.30	3.10
Pond Q	0 – 20	7.00	6.50	5.00	4.50	4.35	4.00	3.60	3.50	3.20	3.10
Dyke Soil Pond P	Surface	2.05	1.95	1.95	1.90	1.90	1.90	-	-	-	1.90

TABLE III

Chemical properties of soil from Buguma fish farm

Sample Location	Depth cm	pH		Organic Carbon %	Total Nitrogen %	Availble Phosphorus ppm	ECm mhos/ cm	Exch. Acidity meq/100g Soil	Exch. Al. meq/ 100g Soil	CEC meq/ 100g Soil	K meq/ 100g Soil	Ca meq/ 100g Soil	mg meq/ 100g Soil	SO₄-S ppm
Sample Location	Depth cm	Wet	Dry	Organic Carbon %	Total Nitrogen %	Availble Phosphorus ppm	ECm mhos/ cm	Exch. Acidity meq/100g Soil	Exch. Al. meq/ 100g Soil	CEC meq/ 100g Soil	K meq/ 100g Soil	Ca meq/ 100g Soil	mg meq/ 100g Soil	SO₄-S ppm
Pond K	0 – 12	7.20	3.70	4.21	0.14	1.75	125.0	16.96	7.89	77	0.52	7.44	31.49	2,700
Pond K	12 – 28	7.50	2.90	4.12	0.13	3.16	60.0	11.58	4.47	53	0.59	2.78	7.10	2,565
Pond K	28 – 57	7.60	3.40	0.17	0.07	7.07	6.0	5.87	4.09	27	0.60	1.25	2.83	1,100
Pond K	57 – 133	7.70	3.60	0.53	0.07	7.08	13.5	4.14	3.71	27	0.62	1.01	2.98	1,182
Pond K	113 – 154	7.60	3.20	1.03	0.06	7.02	7.1	3.49	2.28	27	0.62	0.86	2.88	1,000
Pond K	154 – 169	7.45	3.70	0.77	0.07	7.02	7.1	2.66	2.00	33	0.67	1.06	2.02	1,000
Pond I₁	0 – 30	7.10	3.60	4.38	0.35	7.05	29.2	18.20	4.55	140	0.64	9.10	30.20	2,765
Pond P	0 – 20	6.95	3.10	4.27	0.05	1.40	36.0	21.72	4.47	113	0.62	9.79	26.21	3,000
Pond Q	0 – 20	7.00	3.10	4.35	0.29	1.40	38.0	17.78	3.90	150	0.60	11.14	26.45	2,835
Dyke Soil Pond X	Surface	2.05	1.90	4.42	-	1.75	45.0	48.45	9.88	147	0.56	1.87	13.34	3,000

Oxidisable organic matter expressed as carbon content in the soils often exceeded 4% except in the middle and lower parts of the profile where values ranged between 0.53 – 1.03%. Total nitrogen was quite low in samples from the profile, values ranged between 0.06 – 0.14%, while high values of 0.29% and 0.35& were recorded from ponds Q and I₁ respectively. The high values were probably due to application of fertilizers in these ponds. Available phosphorus was low in all samples analysed, values ranged between 1.40 – 7.02ppm.

Cation exchange capacity (CEC) of the soils was very high. Values higher than 100meq/100g of soil was recorded for ponds P, Q and I₁. These high values might be due to the NaOAC method used for analysis. Mg, Ca and K ions were moderately high with mg ions dominating the complex. Low values of Ca ions were recorded for the middle and lower zones of the soil profile.

High values of sulfate-sulfur (SO₄-S) were recorded for most of the samples. Values ranged between 1000 – 3000ppm. Very high values ranging between 2,832 and 3,000ppm were recorded from the dry dyke soils of production pond P and the bottoms of ponds P and Q. The layers bellow the water table in the soil profile (113 – 169cm) had the lowest concentration of SO₄-S (Table III). Lime requirement was determined using a buffer solution of pH , results indicate the need for approx. 4.1 tonnes of agricultural lime per hectare for neutralization.

DISCUSSION

Results of soil analysis indicate that the soils of the fish farm, Buguma contained substantial amount of sulfidic materials. Oxidation of these materials releases acid into the pond waters resulting in very low pH values. The dykes soils contained abundant jarosite, a potentially dangerous mineral. Hydrolysis of this mineral contributes large amount of acid into pond waters especially after heavy rains. Low yields and fish mortalities are attributed to the lowering of pH in the pond waters.

The commonest management technique used in reducing acidity is by extensive use of lime. Neutralizing the first 10cm of pond bottom and the whole dyke body, requires large quantities of lime. This has however proved uneconomical in many cases because acid sulfate soils have very high residual acidity which could only be satisfied by application of large amounts of lime over a very long period. As indicated, the lime requirement calculated for the soils was about 4.1 tonnes/ha. This value is very high, uneconomical andfar in excess of practical liming rates. In view of this, there is the need to apply other management techniques which have been used in countries having similar problems.

Two general approaches have been proposed for the development and management of fish ponds built on acid-sulfate soils (Singh, 1980; Simpson et al, 1983; Simpson and Pedini, 1985). The first approach (Singh, 1980) recommends the traditional South-east Asian pond design. The ponds are tidal, and management approach involves long period of repeated flushing with seawater, extensive use of lime and large addition of phosphate fertilizer. The second approach (Simpson et al, 1983; Simpson and Pedini, 1985) advocates minimal excavation in order not to expose deep soil layers which may be acidic and avoiding building massive dykes with excavated soil which would then oxidise and create serious acidity reaching problems. This approach also recommends the use of pumps to fill and drain ponds rather than by tidal exchange. The use of agricultural lime is not recommended as a critical step in reducing acidity.

Based on the results of this study and the various approaches recommended in the literature, the following suggestions are considered relevant to the management of acid-sulfate soils of the experimental fish farm at Buguma:

In view of the fact that results indicate that a sulfidic horizon exists quite close to the surface (12cm from pond bottom), minimal soil excavation should be carried out during the process of pond preparation so as not to expose this layer.
After pond preparation, repeated sequence of flushing and draining using tidal waters should be carried out until an ideal pH (6.5 – 7.0) for pond water is got. The use of pumps for draining and filling ponds is not recommended because of extra operating costs.
Systematic collections of water quality data related to acidity impacts especially pH and alkalinity should be carried out. If the pH gets as low as 5, the pond water should be flushed out and exchanged with tidal waters.
Since the dykes were found to contain abundant yellow coloured mottles of jarosite due to exposure, minimal exposure of dyke soil is advocated. Also small amounts of agricultural lime should be scattered on the slopes of the dykes from time to time especially during the rainy season.

ACKNOWLEDGEMENTS

I wish to thank Professor A. G. Ojanuga and Dr. N. O. Isirimah of the Department of Soil Science, Rivers State University of Science and Technology for helpful suggestions and for making their facilities available for this study. Mr. O. Harry of the Soil Science Laboratory, Rivers State University of Science and Technology, assisted with soil analysis.

I am very grateful to Dr. P. Loganathan (RSUST) and Dr. M: N. Kutty (ARAC) for critically reviewing the manuscript. My thanks also go to Mr. J. G. Tobor (NIOMR) and Mr. M. A. Afinowi for stimulating and encouraging this study.

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